1. Field of the Invention
This invention relates generally to microscope viewing stages and more particularly to adjustable viewing stages for particle beam or laser beam microscopes.
2. Description of the Prior Art
Semiconductor wafers can be viewed with Scanning Electron Microscopes to look for defects that might degrade or impair the functioning of integrated circuits made from the wafer. A Scanning Electron Microscope (SEM) sweeps a beam of electrons across the wafer which stimulates the emission of secondary electrons from the wafer surface. The secondary electrons are detected to develop an image of the surface of the wafer on a Cathode Ray Tube (CRT) monitor. Unlike optical microscopes, the electron microscope has electrically controlled, variable focus lenses and thus the distance between the specimen and the objective lens can vary.
In the past, wafers have been primarily inspected from a plan view substantially normal to the plane of the wafer. Unfortunately, a plan view of the wafer does not allow an operator to properly view undercuts and other features hidden from a vertical view.
A solution to this problem, addressed in an abandoned copending application Ser. No. 491,516, filed May 24, 1983 allowed the wafer to be tilted and rotated relative the lens so that an operator can see oblique elevational views of the wafer's surface. If the wafer is rotated or tilted eucentrically around the inspection point on the wafer (i.e. the axis of rotation or tilt passes through the inspection point), the inspection point will remain in the field of view and the focus will be maintained. On the other hand, if the wafer is rotated or tilted non-eucentrically (i.e. the axis of rotation or tilt does not pass through the inspection point), the inspection point will no longer be within the field of view.
Stage assemblies capable of eucentric rotation and tilt tend to be more complex mechanically and thus more expensive than stage assemblies that rotate or tilt non-eucentrically. For example, a stage assembly capable of eucentric tilt needs to be able to move in X, Y, T, and Z directions to minimize working distances at any given tilt angle. On the other hand, a non-eucentric stage assembly needs only to move in the X, Y, and T directions to minimize working distances, and thus eliminates the cost of Z direction adjustment mechanisms and controllers. Furthermore, the rotation mechanism of eucentric stage assemblies are complicated by the fact that they must be capable of rotating an entire X and Y carriage assembly rather than just a wafer pedestal, as is the case with a non-eucentric stage assembly.